Crista (SPOILERS)

If my subtitles were correct, after Stasi searches his house, Dreyman discuss this with Hauser and the other guy. He says that if Crista was really the one to rat him out, and she didn't say where the typewriter was hidden then "She is our guardian angel".

My interpretation is, since she accused him and they found nothing, it clears him from any future suspicion. Since the Stasi already searched his house and didn't find anything, they would not have any reason to suspect him.

Did Crista really "rat out" her boyfriend the first time?

I thought that it was only because she started crying (after the remark that only the identity of the writer of the article could save her) that the Stasi took her crying as a hint that her boyfriend was the writer of the article. I think that she did not explicitly say it then?

After all, she only became official Stasi collaborator "IM Marta" after the second time, didn't she? If she had explicitly "ratted out" her boyfriend the first time, then why would she not have become "IM Marta" the first time?

WoW this woman was put in a impossible position. As an aspiring actress having to sleep with that dirty old man Hempf just to continue her passion and talent is already sucking life out of your soul! As portrayed in pill addiction!
But what Stasi did on top is unimaginable just because her Director/BF is Doing a Little Something. JUST an Article. Ratting out like school kids in no real need is no comparison to Stasi Jail. The desperation on her face is well acted 'to get'. We don't know how many hours she refused a testimony. Maybe 48 already. Dreyman mentioned she is gone unknown for days shortly before returning. Beginning of movie is explained how they psychological break people to gain the information they want when first refusing. The surrogate interviews are really clever build up. Anyone would have. And that was actual real life. No fiction. Most all had IM status. Everyone spying on everyone. Some took it more serious than others of course, just wanting out of jail in the first place! That was the real terror. Friends, colleagues, neighbors no-one can be trusted with thoughts.

Most important for police is always to find the actual weapon. Without it making each case severely weaker. It is like that technically legally. Accept it! OJs alleged glove didn't fit his hand. Of course hands can swell with fly stitches (/artificial injections), working out with Hand-Trainers e.g..... but the bloody gloves didn't fit. Soo...
As long as they don't find the unregistered typewriter matching article letters font found in his (hidden) possession there is to weak proof for a real court case. It's just Words by another Person with no real proof. Accusation vs Accusation. He is a public person. Not just in DDR but at very least of significance in BRD, latest by his article. This makes him a 100% Proof-Case or 0 not at all.
Of course Stasi would have kept on surveilling him for the rest of his life if the Wall hadn't come down short after.